Method and apparatus for projection type mask alignment

ABSTRACT

A mask alignment method of the projection type is disclosed which is based upon a fact that the exit pupil of a projection lens is actually positioned at a finite distance, and wherein a first wafer alignment pattern including a line segment and a second wafer alignment pattern including another line segment are formed on a wafer in those radial directions from the optical axis of a projection lens which intersect with each other approximately at a right angle. A first mask alignment pattern including a line segment and a second mask alignment pattern including another line segment are formed respectively at those positions on a mask which optically correspond to respective positions of the first and second wafer alignment patterns through the projection lens. The optical image of the first wafer alignment pattern superposed on the optical image of the first mask alignment pattern by the action of the projection lens falls on an image pickup device or element, the optical image of the second wafer alignment pattern superposed on the optical image of the second mask alignment pattern by the action of the projection lens falls on another image pickup device or element, the relative displacement between the wafer and the mask is determined by the video signals delivered from the image pickup devices or elements, and the wafer and the mask are aligned with each other so as to reduce the relative displacement to zero.

BACKGROUND OF THE INVENTION

The present invention relates to a method and an apparatus forprojection type mask alignment, and more particularly to a method and anapparatus for reduction-projection type mask alignment.

A conventional exposure apparatus of reduction-projection type has sucha structure as disclosed in U.S. Pat. No. 4,153,371. That is, in theconventional exposure apparatus, as is shown in FIGS. 1A, 1B and 1C ofthe accompanying drawings, a mask 1 is disposed at a distance from awafer 3 with a projection lens 2 placed therebetween, and a condenserlens 4 is disposed above the mask 1. The mask 1 is illuminated by anexposure light emitted from an exposure light source (not shown) throughthe condenser lens 4, and a mask pattern 5 formed on the mask 1 isprojected through the projection lens 2 onto the chips 8 of the wafer 3in the form of a reduced image 27. At that time, in order to align themask 1 with the directions of step and repeat movement of awafer-feeding table for carrying the wafer 3 and to place the mask 1 onthe origin of an absolute coordinate, a relative displacement between amark (or a cross-shaped alignment pattern) formed in a microscope (notshown) for positioning the mask 1 and an alignment pattern 6 formed onthe mask 1 and having the shape of is detected automatically or by nakedeyes on the X and Y axes of the absolute coordinate which are coincidentwith the directions of movement of the wafer-feeding table for carryingthe wafer 3, and then an X-axis feed table, a Y-axis feed table and arotary table, all of which are used to carry the mask 1, are subjectedrespectively to a fine displacement in the direction of X axis, a finedisplacement in the direction of Y axis and an angular displacement θ,in accordance with the above relative displacement.

In the above-mentioned exposure apparatus of projection type, accordingto a conventional mask alignment apparatus of projection type foraligning the mask 1 and the wafer 3 with each other, a chromium film isdeposited on the surface of the peripheral portion of the mask 1 byevaporation technique to form a mask alignment pattern 7 including atransparent portion having a size of about 400 μm×400 μm. On the otherhand, a wafer alignment pattern comprised of a cross-shaped groovehaving a width of about 5 μm is formed in the surface of the wafer 3which is coated with a photoresist film. Further, the mask alignmentapparatus is equipped with a first and a second optical system and adetection system. The first optical system is made up of a mercury lamp13 emitting the same light as the exposure light, an interference filter14, condenser lenses 15 and 16, a field diaphragm 17, a semi-transparentmirror 12, and a reflection mirror 11. The second optical systemincludes a reflection mirror 10, and illuminates the mask alignmentpattern 7 in a direction different from that of the first opticalsystem. The detection system includes an objective lens 18,semi-transparent mirrors 19 and 20, an image rotating prism 21, areflection mirror 22, a plate provided with a slit 23, andlight-detecting elements 24 and 25.

As is known in the prior art, the wafer 3 is mounted previously on acassette jig (not shown) in another station, and is subjected to acoarse alignment. The cassette jig is placed on the wafer-feeding table(not shown) which is driven by a step and repeat operation, in such amanner as to be put in contact with positioning pins provided on thewafer-feeding table. The wafer-feeding table is moved in a positivedirection along the X axis, for example, by a distance equal to N×P, inorder to place the leftmost chip 8 of the wafer 3 on the optical axispassing the center of the reduction-projection lens 2, where Nrepresents the number of chips and P a pitch of chips, namely a lengthof a unit step. The light emitted from the mercury lamp 13, which hasthe same wavelength components as the exposure light, illuminates themask alignment pattern 7 on the mask 1 through the interference filter14, the condenser lenses 15 and 16, the field diaphragm 17, thesemi-transparent mirror 12 and the reflection mirror 11. The lighthaving passed through the mask alignment pattern 7 travels toward thecenter A of the entrance pupil of the projection lens 2 as the incidentlight. The light having passed through the reduction-projection lens 2travels toward the wafer alignment pattern 9 on the chip 8 from thecenter B of the exit pupil of the reduction-projection lens 2, and formsa reduced optical image 7' of the mask alignment pattern 7 on the waferalignment pattern 9. The reflected light from the wafer alignmentpattern 9 passes through the reduction-projection lens 2, and thereforethe patterns 7 and 9 are again projected onto the surface of the mask 1.The reflected light having passed through the transparent portion of themask alignment pattern 7 travels toward the slit 23 through thereflection mirror 11, the semi-transparent mirror 12, the objective lens18, the semi-transparent mirrors 19 and 20, the image rotating prism 21and the reflection mirror 22 to form on the slit plane such an opticalimage having an interference fringe as shown in FIG. 2A. The opticalimage of the alignment patterns 7 and 9 as shown in FIG. 2A is convertedby the light-detecting elements 24 and 25 into such signals as shown inFIGS. 2B and 2C, when the slit 23 is subjected to reciprocating motion.The relative displacement Δx₁ in the direction of X axis between thepatterns 7 and 9 and the relative displacement Δy₁ in the direction of Yaxis between the patterns 7 and 9 are detected from the signals shown inFIGS. 2B and 2C, and then stored into a memory. In more detail, thedetection of the relative displacement Δx₁ and Δy₁ is made in thefollowing manner. As shown in FIGS. 2A, 2B and 2C, the region of themask 1 surrounding the transparent portion has a bright level since thesurrounding region is illuminated with the second optical systemincluding the reflection mirror 10. At the same time, the reflectedlight from the wafer 3 has a weak intensity, and therefore thetransparent portion has a less bright level, so that the position of thetransparent portion of the mask alignment pattern 7 is determined from apair of clear boundaries between the transparent portion and the regionsurrounding the transparent portion, and the relative displacement Δx₁and Δy₁ are determined by the position of the transparent portion andthe center of the wafer alignment pattern 9.

Next, the wafer-feeding table which is driven by step and repeatoperation, is moved in a negative direction along the X axis, forexample, by a distance of 2N×P, to position the rightmost chip 8 of thewafer 3 on the optical axis. Then, the light emitted from the mercurylamp 13 and having the same wavelength components as the exposure lightis directed to the mask alignment pattern 7 and the wafer alignmentpattern 9 to form the optical images of the patterns 7 and 9 on the slitplane by the reflected light from the wafer 3. In a similar manner tothat above-mentioned, the relative displacement Δx₂ in the direction ofX axis between the patterns 7 and 9 and the relative displacement Δy₂ inthe direction of Y axis between the patterns 7 and 9 are detected on thebasis of the signals which are delivered from the light detectingelements 24 and 25 when the slit plane is subjected to reciprocatingmotion, and the detected displacement Δx₂ and Δy₂ are stored into amemory.

Next, the wafer-feeding table carrying the wafer 3 is slightly rotatedby an angle θ which is equal tp (Δy₁ -Δy₂)/2N×P, to make the directionsin which chips 8 have already been arranged in the wafer 3 respectivelycoincide with the directions of X and Y axes which are equal to thedirections of step and repeat movement. With respect to the direction ofY axis, the mask-feeding table carrying the mask 1 is moved in thedirection of Y axis by a distance equal to Δy₁ (or Δy₂). Further, withrespect to the direction of X axis, the mask-feeding table is moved inthe direction of X axis by a distance equal to Δx₁ (or Δx₂). Thus, themask 1 and the wafer 3 are aligned with each other.

After the alignment between the mask 1 and the wafer 3 has been achievedin the above manner, the wafer-feeding table which is driven by step andrepeat operation, is moved by a length equal to P in the directions of Xand Y axes, and the wafer 3 is illuminated by the exposure light everytime, the wafer-feeding table is moved in the above directions. Thus, alarge number of chips which are arranged on the wafer 3 in the form of acheckerboard, can be exposed and printed.

The illumination light travels along such an optical path as shown inFIG. 3, and is reflected from the wafer 3 as shown in FIG. 4. In moredetail, the illumination light 30 having passed through the objectivelens 18 is reflected from the mirror 11, passes through the transparentportion of the mask alignment pattern 7, and then travels toward thecenter A of the entrance pupil of the projection lens 2. Theillumination light 31 having passed through the lens 2 travels in thedirection from the center B of the exit pupil of the lens 2 to the wafer3, and the wafer 3 is illuminated by the light 31. In the presentoptical design technique, it is impossible to make the incident angle θof the illumination light 31 equal to zero. The illumination light 31 isreflected from the wafer alignment pattern 9 in such a manner as shownin FIG. 4. In more detail, the wafer alignment pattern 9 having the formof a cross is made up of a cross-shaped groove formed in the surface ofthe silicon substrate 9a and having a depth of 1 to 2 μm, and aphotoresist film 9b coated on the surface of the substrate 9a. Whenthose steps on both sides of the groove which are formed along thedirection of Y axis, are illuminated by the light 31, the reflectedlight 32a from one of the steps and the reflected light 32b from theother step travel in respective directions which are different from eachother and asymmetrical with respect to the plane which is defined by theoptical axis of the projection lens 34 and the light 31. Both thereflected light 32a and the reflected light 32b pass through theprojection lens 2, and form an reflected image 9' of the wafer alignmentpattern 9 on the surface of the mask 1. However, all of each of thereflected light 32a and the reflected light 32b do not enter into theprojectionlens 2, but only a part of each light 32a or 32b passesthrough a diaphragm 2a in the lens 2, and serves to form the reflectedimage 9'. The reflected light 32a and the reflected light 32b aregreatly different in quantity of light capable of passing through theprojection lens 2 from each other, and therefore the signal obtainedfrom the light-detecting element by the reciprocating motion of the slitplane has such an asymmetric form as shown in FIG. 2B. As a result, itis different to detect the center of the wafer alignment pattern in thedirection of X axis, with high accuracy, and therefore it is impossibleto align the wafer with the mask with high accuracy in the direction ofX axis.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide a method and anapparatus for projection-type mask alignment in which a mask and a wafermay be precisely aligned to each other with high accuracy by the use ofa projection lens whose exit pupil is spaced apart from the lens at afinite distance.

In order to attain the above object, in a mask alignment methodaccording to the present invention which is based upon a fact that theexit pupil of a projection lens is actually positioned at a finitedistance from the lens, a first wafer alignment pattern including a linesegment and a second wafer alignment pattern including another linesegment are formed within a projection region of a projection lens on awafer in those radial directions from the optical axis of the projectionlens which intersect with each other at a right angle, that is, theextension line of the center line of the first wafer alignment pattenand that of the second wafer alignment pattern intersect with each otherat a right angle on the optical axis of the projection lens. Further, afirst mask alignment pattern including a line segment and a second maskalignment pattern including another line segment are formed respectivelyat those positions on a mask which optically correspond to respectivepositions of the first and second wafer alignment patterns through theprojection lens. The optical image of the first wafer alignment patternsuperposed on the optical image of the first mask alignment pattern bythe action of the projection lens falls on an image pickup device, andthe optical image of the second wafer alignment pattern superposed onthe optical image of the second mask alignment pattern by the action ofthe projection lens falls on another image pickup device. The relativedisplacement in the directions of X and Y axes between the wafer and themask is determined by the video signal delivered from each of the imagepickup devices, and the wafer and the mask are aligned with each otherso as to reduce the relative displacement to zero. Further, a feature ofthe present invention resides in a mask alignment apparatus ofprojection type for realizing the above mask alignment method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view showing a structure of a conventional maskalignment apparatus of reduction-projection type.

FIG. 1B is an enlarged view of the mask alignment pattern shown in FIG.1A.

FIG. 1C is a perspective view for showing a wafer alignment patternformed on each chip of the wafer shown in FIG. 1A, together with theoptical image of the mask alignment pattern.

FIG. 2A shows optical images of the mask and wafer alignment patternsformed on the slit plane shown in FIG. 1A, together with the scanningslit.

FIG. 2B shows the waveform of the video signal detected from the opticalimages shown in FIG. 2A by the light-detecting element for displacementin the direction of X axis.

FIG. 2C shows the form of the video signal detected from the opticalimages shown in FIG. 2A by the light-detecting element for displacementin the direction of Y axis.

FIG. 3 shows the optical path of the illumination light in the apparatusshown in FIG. 1A.

FIG. 4 shows the optical path of the light reflected from the waferalignment pattern formed in each chip of the wafer shown in FIG. 1A.

FIG. 5 is a perspective view showing an embodiment of a mask alignmentapparatus of reduction-projection type according to the presentinvention.

FIG. 6 is a view for showing the mask and wafer alignment patterns in astate that the mask shown in FIG. 5, the reduction-projection lens shownin FIG. 5, and the leftmost chip of the wafer shown in FIG. 5 overlapeach other.

FIG. 7 shows optical images formed at respective slit portions of thescanning plates shown in FIG. 5.

FIG. 8(a) shows that the optical images shown in FIG. 7 is scanned bythe slit, and FIG. 8(b) shows the waveform of the video signal deliveredfrom each of the light-detecting elements shown in FIG. 5.

FIG. 9 shows mask and wafer alignment patterns, which are formed atother positions than the positions of the mask and wafer alignmentpatterns shown in FIG. 6.

FIG. 10 shows mask and wafer alignment patterns, which are formed atother positions than the positions of the mask and wafer alignmentpatterns shown in FIGS. 6 and 9.

FIGS. 11A and 11B show mask and wafer alignment patterns, which differin shape from those shown in FIGS. 7 and 8.

FIG. 12 shows a case where four chips each having a size of 5 mm×5 mmare printed by a single exposure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 5 shows the construction of an embodiment of a mask alignmentapparatus of reduction-projection type which is usable in realizing amask alignment method according to the present invention. In a differentstation, the wafer 3 is mounted on a cassette jig (not shown) having aprecisely-finished side face and is subjected to a coarse alignment. Thecassette jig carrying the wafer 3 is placed on a wafer-feeding table 28in such a manner as being kept in contact with and fixed by positioningpins 29 fixed to the upper surface of the wafer-feeding table 28. Thus,the wafer 3 is coarsely aligned. The wafer-feeding table 28 includes anX-axis feed table 28a, a Y-axis feed table 28b and a rotary (or θ) table28c. The tables 28a and 28b conduct the step and repeat movement. Alarge number of chips are arranged on the wafer 3 in the form of acheckerboard. As is shown in FIGS. 5 and 6, a rectilinear waferalignment pattern 35b is formed on the leftmost part of each chip 8 inthe direction parallel to the X axis and passing the optical axis 34 ofthe reduction-projection lens 2 (or the center of each chip), andanother rectilinear wafer alignment pattern 35a is formed on thelowermost part of each chip 8 in the direction parallel to the Y axisand passing the optical axis 34. The mask 1 includes therein anintegrated circuit pattern 5, transparent windows each having the formof a square for aligning the mask 1 with the wafer 3, namely, maskalignment patterns 7b and 7a formed respectively on the rightmost partof the mask 1 in the direction parallel to the X axis and passing theoptical axis 34 of the reduction-projection lens 2 and the uppermostpart of the mask 1 in the direction parallel to the Y axis and passingthe optical axis 34, and a pair of alignment marks 6 formed in twocorners of the mask 1 for positioning the mask 1 with respect to theabsolute coordinate (or reference coordinate) which corresponds to thedirections of step and repeat movement of the mask-feeding table. Themask 1 having the above patterns and marks is mounted on a mask-feedingtable which is provided with a through hole at the central portionthereof. The alignment marks 6 are observed by means of a pair ofmicroscopes (not shown). The relative displacement between eachalignment mark and the mark (showing the absolute coordinate) formed ineach microscope is detected by optical detectors or naked eyes and themask-feeding table carrying the mask 1 is moved automatically ormanually so as to reduce the relative displacement to zero. Thus, themask 1 is positioned with respect to the absolute coordinate with highaccuracy. In more detail, the positioning of the mask 1 is conductedwith very high accuracy with respect to the directions of step andrepeat movement of the X-axis and Y-axis feed tables 28a and 28b(namely, the directions of X and Y axes) and with respect to thereference position (namely, the origin of the coordinate).

Next, explanation will be made on the alignment of the wafer 3 with themask 1, namely, the alignment of the wafer 3 with the absolutecoordinate (or reference coordinate). Referring to FIG. 5, there areprovided two detection systems 45a and 45b, which correspond to thealignment patterns 7a and 35a and the alignment patterns 7b and 35b,respectively. The detection system 45a (or 45b) is made up of an opticalfiber 48a (or 48b) for guiding the light which illuminates the maskalignment pattern 7a (or 7b), a condenser lens 47a (or 47b), areflection mirror 10a (or 10b), an optical fiber 36a (or 36b) forguiding the light which is emitted from a mercury lamp and has the samewavelength components as the exposure light, a semi-transparent mirror12a (or 12b), an objective lens 18a (or 18b), a reflection mirror 11a(or 11b), a lens 37a (or 37b), a reflection mirror 38a (or 38b), acondenser lens 40a (or 40b), a light-detecting element 24 (or 25) suchas a photomultiplier, a scanning plate 39a (or 39b) provided with a slit23a (or 23b) and conducting the reciprocating motion, a flat spring 44a(or 44b) for supporting the scanning plate 39a (or 39b), a galvanometer41a (or 41b) conducting the rotational vibration, a lever 42a (or 42b)fixed to the output shaft of the galvanometer 41a (or 41b), and a pin43a (or 43b) which is fixed to one end of the lever 42a (or 42b) and iskept in contact with the scanning plate 39a (or 39b).

The exposure portion of the embodiment shown in FIG. 5 is made up of alight source 54 such as a mercury lamp, a filter 53, condenser lenses 52and 51, a reflection mirror 50, and a condenser lens 4. Further, a pairof optical systems are employed to illuminate only the mask alignmentpatterns 7a and 7b. In more detail, the optical system for illuminatingthe pattern 7a (or 7b) includes the optical fiber 48a (or 48b), thecondenser lens 47a (or 47b), the reflection mirror 46a (or 46b), and thereflection mirror 10a (or 10b). That portion of the mask alignmentpattern 7a (or 7b) which surrounds the transparent window, isilluminated with the light from the above optical system, and thereforeis put in a high level in the signal delivered from the light-detectingelement 24 of the detection system 45a (or the light-detecting element25 of the detection system 45b), as shown in FIG. 8(b). Accordingly, thedistance M₁ between a reference position and one edge of the window andthe distance M₂ between the reference position and the other edge of thewindow can be determined with high accuracy. Further, the light forilluminating the wafer alignment pattern 35a (or 35b) is directed to thecenter A of the entrance pupil of the reduction-projection lens 2 bymeans of the optical fiber 36a (or 36b), the semi-transparent mirror 12a(or 12b), the objective lens 18a (or 18b), and the reflection mirror 11a(or 11b). The illumination light thus directed impinges upon and isreflected from the wafer alignment pattern 35a (or 35b). The reflectedlight travels in the reverse direction on the optical path of theincident light, and then reaches the slit 23a (or 23b) through the lens37a (or 37b) and the reflection mirror 38a (or 38b). The focus of theobjective lens 18a (or 18b) is placed upon the mask alignment pattern 7a(or 7b), and respective optical images of the alignment patterns 7a and35a (or 7b and 35b) are formed on the plane containing the slit 23a (or23b). Further, the light guided by each of the optical fibers 36a, 36b,48a and 48b has the same wavelength components as the exposure light,and therefore the focus of the reduction-projection lens can beprevented from becoming vague due to chromatic aberration.

The embodiment shown in FIG. 5 is operated as follows. At first, theX-axis feed table 28a is moved to the right from the origin of thecoordinate (which is placed on the optical axis 34 of thereduction-projection lens 2) by a length of N×P in accordance with acommand from a control unit (not shown), where N indicates the number ofchips and P a pitch of chips. The above operation is similar to that inthe case where the integrated circuit pattern is first printed. Thelength, which is equal to N×P, is determined with high accuracy by theuse of a measuring instrument employing laser light. When the X-axisfeed table 28a has been moved as above, a chip 8x₁, is placed upon theoptical axis 34. At that time, the optical image of the rectilinearwafer alignment pattern 35a of the chip 8x₁, and the optical image ofthe mask alignment pattern 7a are combined with each other, as shown inFIG. 7. Similarly, the optical image of the pattern 35b of the chip 8x₁,and the optical image of the pattern 7b are, as shown in FIG. 7,combined with each other. Since the rectilinear wafer alignment patterns35a and 35b are placed on the straight lines which are radially extendedfrom the center of the chip (which is placed on the optical axis 34),the light reflected from one of the facing steps of the rectilinearpattern 35a or 35b and the light reflected from the other of the facingsteps travel symmetrically with respect to the plane defined by theoptical axis 34 of the projection lens 2 and the incident light beam,even if the illumination light impinges upon the wafer alignment patternin the direction from the center B of the exit pupil which is placed ata finite distance, to the wafer alignment pattern, namely, in thedirection producing an incident angle θ. Thus, the light reflected fromone of the facing steps of the wafer alignment pattern and the lightreflected from the other step are equal in quantity of light to eachother when they pass through the projection lens 2, and therefore it ispossible to obtain such a symmetrical signal as shown in FIG. 8(b).Accordingly, the relative displacement Δx₁ in the direction of X axisbetween the mask 1 and the chip 8x₁ can be determined with high accuracyby detecting the optical images of the alignment patterns 7a and 35a bythe light-detecting element 24 through the slit 23a conducting thereciprocating motion. The mask alignment pattern 7b is placed at theposition which is obtained by rotating the position of the pattern 7around the optical axis 34 by an angle of 90°, and the wafer alignmentpattern 35b is placed at the position obtained by rotating the positionof the pattern 35a round the optical axis 34 by an angle of 90°. Likethe relative displacement Δx₁, the relative displacement Δy₁ in thedirection of Y axis between the mask 1 and the chip 8x₁ is obtained withhigh accuracy by detecting the optical images of the alignment patterns7b and 35b by the light-detecting element 25 through the slit 23bconducting the reciprocating motion.

Next, the illumination of the alignment patterns with the illuminationlight is stopped, and then the X-axis feed table 28a is moved to theleft by a length of 2N×P in accordance with a command from the controlunit (not shown). The above length is determined with high accuracy bythe use of the measuring instrument employing laser light. Then, a chip8x_(n) is placed on the optical axis 34. At that time, as is shown inFIG. 7, the optical image of the rectinear wafer alignment pattern 35aof the chip 8x_(n) and the optical image of the mask alignment pattern7a are combined with each other, and the optical image of therectilinear wafer alignment pattern 35b of the chip 8x_(n) and theoptical image of the mask alignment pattern 7b are combined with eachother. In a similar manner to the previously-mentioned, the alignmentpatterns are illuminated with the illumination light, and the opticalimages of the alignment patterns are detected by the detectors 45a and45b. Thus, the relative displacement Δx₂ in the direction of X axisbetween the mask 1 and the chip 8x_(n) and the relative displacement Δy₂in the direction of Y axis are obtained with high accuracy.

An angle θ equal to (Δy₂ -Δy₁)/2NP indicates an angular displacement ofthe wafer 3 in the rotational direction. The rotary table 28c carryingthe wafer 3 is rotated so as to make the angle θ equal to zero, and thusthe relative displacement in the rotational direction between the mask 1and the wafer 3 is reduced to zero. Next, the reference control signalstored in a control circuit for causing the X-axis feed table 28a andthe Y-axis feed table 28b to conduct the step and repeat movement iscorrected so as to eliminate the error Δx₁ (or Δx₂) in the direction ofX axis and the error Δy₁ (or Δy₂) in the direction of Y axis. Then, thewafer 3 can conduct the step and repeat movement while being alignedwith the mask 1. Since the wafer 3 is subjected to such chemicaltreatment as diffusion, expansion and contraction are generated in thewafer 3. In order to solve this problem, chips 8y₁ and 8y.sub. n whichare arranged in the direction of Y axis, are further examined in thepreviously-mentioned manner, and the relative displacement (Δx₃, Δy₃)with respect to the chip 8y₁ and the relative displacement (Δx₄, Δy₄)with respect to the chip 8y_(n) are determined. The pitch P of the stepand repeat movement is corrected on the basis of the expansion andcontraction (Δx₂ -Δx₁) of the wafer 3 in the direction of X axis and theexpansion and contraction (Δy₄ -Δy₃) in the direction of Y axis. Then,the mask 1 and the wafer 3 can be aligned with each other with highaccuracy.

Referring to FIG. 9, rectilinear wafer alignment patterns 35c and 35dare formed in two corners of each chip 8 of the wafer 3 in such a mannerthat the extension line of the center line of the pattern 35c and thatof the pattern 35d intersect at a right angle at the center of the chip.Further, mask alignment patterns 7c and 7d, each of which includes atransparent window having the form of a square, are formed in twocorners of the mask 1. The mask and wafer having the alignment patterns35c, 35d, 7c and 7d can produce the same effect as the embodiment shownin FIGS. 5 and 6. In this case, however, the relative displacement inthe directions of X and Y axes cannot be determined directly, but therelative displacement directly detected has to be converted into an Xcomponent and a Y component.

In the foregoing embodiments, a pair of rectilinear wafer alignmentpatterns are formed on each chip 8 of the wafer 3 in such a manner thatrespective center lines of the patterns intersect at a right angle atthe center of the chip. However, it is not always required that thepatterns intersect at a right angle. Referring to FIG. 10, rectilinearwafer alignment patterns 35e and 35f may be arranged radially from thecenter of each chip with an angle of 90°±θ° made therebetween, throughthe accuracy in alignment may be reduced to some extent. In this case,mask alignment patterns 7e and 7f are formed at those positions on themask 1 which correspond to the wafer alignment patterns 35e and 35f,respectively. In a case where the angle θ is greater than 45°, forexample, the accuracy, with which the relative displacement in thedirection of X axis is detected, is decreased. For this reason, it ispreferable to make the angle θ less than 30°.

In the foregoing embodiments, the rectilinear alignment patterns areformed on the wafer 3, and the square alignment patterns are formed onthe mask 1. However, as shown in FIG. 11A, square alignment patterns 35'may be formed on the wafer 3. Further, as shown in FIG. 11B, rectilinearalignment patterns 7' may be formed on the mask 1. In either case,however, the respective center lines of the wafer alignment patternshave to be directed to the center of the chip.

In the foregoing description, explanation has been made on cases whereone of the chips 8 each having a size of 10 mm×10 mm is printed by asingle exposure. However, it is also possible that a circuit patterncorresponding to four chips is formed on the mask 1 and four chips eachhaving a size of 5 mm×5 mm are printed by a single exposure. In thiscase, the rectilinear wafer alignment patterns 35a and 35b may be formedat every four chips in such a manner as being directed to the center offour chips, and the mask alignment patterns 7a and 7b each having theform of a square may be formed on the mask 1 in such a manner as beingarranged radially with respect to the optical axis 34. It is notnecessary for the wafer alignment patterns 35a and 35b to be formed oneach chip.

In the foregoing embodiments, a pair of detection units are employed.However, the scanning plate, the flat spring, the drive mechanism forconducting the reciprocating motion, and so forth can be commonly used,when an image rotator is employed.

Further, a self-scanning type image pickup element (or a linear imagesensor) may be employed as the detection unit.

In the foregoing description, explanation has been made on the alignmentmethod in which a reduction-projection lens is employed. However, aprojection lens having a reduction ratio of 1/1 also produces the samephenomena as the reduction-projection lens, since it is impossible toplace the exit pupil of the lens having a reduction ratio of 1/1 at theinfinite point. Accordingly, the present invention is applicable to analignment method in which a projection lens having a reduction ratio of1/1 is employed.

As has been explained hereinbefore, according to the present invention,the light reflected from a wafer alignment pattern is prevented fromhaving an asymmetric intensity distribution, which is produced due tothe fact that the direction from the exit pupil of areduction-projection lens to the wafer alignment pattern is notperpendicular to the surface of the wafer. As a result, the relativedisplacement in the directions of X and Y axes between a mask and thewafer can be detected with high accuracy. That is, a relativedisplacement less than 0.1 μm can be detected with the present inventionwhen a mask and a wafer are aligned with each other. While, the relativedisplacement detectable with prior art lies within a range of 0.5 to 1μm.

We claim:
 1. A mask alignment method of projection type comprising thesteps of:disposing a wafer in a region onto which a mask is projected bya projection lens, said wafer including first and second wafer alignmentpatterns formed thereon, said first wafer alignment pattern having atleast a first line segment, said first line segment being formed at afirst position on said wafer and having a predetermined length in afirst direction with first and second step edges extending parallel toone another along said predetermined length, said second wafer alignmentpattern having at least another first line segment, said another firstline segment being formed at a second position different than said firstposition on said wafer and having a predetermined length in a seconddirection approximately perpendicular to said first direction with saidfirst and second step edges extending parallel to one another along saidpredetermined length, said first and second wafer alignment patternshaving respective pattern center axes, said pattern center axes beingapproximately parallel to said first and second line segments,respectively, said first and second positions being determined so as tocause directions along which each of said pattern center axes lies tointersect with the optical axis of said projection lens; projectingfirst and second mask alignment patterns of said mask onto said firstand second wafer alignment patterns, respectively, said first and secondmask alignment patterns being laid respectively upon said first andsecond wafer alignment patterns when said mask is projected onto saidwafer by said projection lens, each of said first and second maskalignment patterns including at least a second line segment having apredetermined length, said first and second mask alignment patternsbeing formed at such positions on said mask as to make each of saidsecond line segments parallel to a corresponding one of said first linesegments when said mask is correctly aligned with said wafer; andaligning said mask and said wafer with each other by the use of opticalimages of said first and second wafer alignment patterns and said firstand second mask alignment patterns, said optical images being formed andconverted into video signals, a relative displacement between each ofsaid first line segments and a corresponding one of said second linesegments being determined by a video signal corresponding to said firstline segment and a video signal corresponding to said second linesegment, said relative displacement being reduced to align said waferwith said mask, wherein, during the course of said adjusting, equalamounts of said projected light will be reflected off the first andsecond step edges of each of said first and second wafer alignmentpatterns due to the positional relationship of the longitudinal centeraxes of the first and second wafer alignment patterns to the opticalaxis of the projection lens.
 2. A method for alignment of a wafer with amask using light-projection through a projection lens, said waferincluding a plurality of exposure sections, each including predeterminedcircuit patterns, said method comprising the steps of:preliminarilyforming on each of said exposure sections of said wafer at least firstand second wafer alignment patterns each including at least onelongitudinal mark at the periphery of the circuit patterns includedtherein, each said longitudinal mark including first and second stepedges extending parallel to one another along the length of saidlongitudinal mark, and forming on the mask first and second maskalignment patterns, said first and second wafer alignment patterns andsaid first and second mask alignment patterns being formed in such apositional relationship that when said wafer is disposed at apredetermined region in the path of the light projection, longitudinalcenter axes of said first and second wafer alignment patterns extendsubstantially towards the optical axis of said projection lens, and thatwhen said wafer is positioned in alignment with said mask, longitudinalcenter axes of optical images of said first and second wafer alignmentpatterns projected on said mask through said projection lens are alignedwith the longitudinal center axes of said first and second maskalignment patterns, respectively, disposing the wafer in saidpredetermined region; projecting a light through said mask and saidprojection lens onto said wafer thereby forming optical images of saidfirst and second mask alignment patterns on said wafer; producingelectrical signals representative of positional relationships betweensaid first wafer alignment pattern and said first mask alignment patternand that beween said second wafer alignment pattern and said second maskalignment pattern; and adjusting the position of said wafer relative tothe position of said mask so as to change said electrical signalstowards predetermined conditions which exist when the longitudinalcenter axes of said first and second mask alignment patterns are alignedwith each other, respectively, wherein, during the course of saidadjusting, equal aamounts of said projected light will be reflected offthe first and second step edges of each of said first and second waferalignment patterns due to the positional relationship of thelongitudinal center axes of the first and second wafer alignmentpatterns to the optical axis of the projection lens.
 3. A mask alignmentmethod of projection type according to claim 2 wherein said first andsecond wafer alignment patterns comprise a line pattern, respectively.4. A mask alignment method of projection type according to claim 1 or 2,wherein said first and second mask alignment patterns comprise a squarepattern, respectively.
 5. A mask alignment method of projection typeaccording to claim 1 or 2, wherein a reduction-projection lens is usedas said projection lens.
 6. A mask alignment apparatus of projectiontype comprising:a wafer-feeding table for carrying a wafer, said waferbeing provided with wafer alignment patterns formed in at least twoportions on said wafer, said wafer alignment patterns including linesegments, respectively, said line segments being arranged radially withrespect to the optical axis of a projection lens, the perpendicular fromone of said wafer alignment patterns to said optical axis making anangle approximately equal to 90° with respect to the perpendicular fromthe other wafer alignment pattern to said optical axis, wherein each ofsaid line segments includes first and second step edges extendingparallel to one another along the length of said line segment; amask-feeding table disposed above said wafer-feeding table for carryinga mask, said mask being provided with mask alignment patterns formed inat least two portions on said mask and with a circuit pattern formed onsaid mask, said mask alignment patterns including line segments,respectively, the positions of said mask alignment patterns opticallycorresponding to the positions of said wafer alignment patterns throughsaid projection lens, respectively; said projection lens being disposedbetween said wafer-feeding table and said mask-feeding table forprojecting said patterns formed on said mask onto said wafer; aplurality of units for scanning and detecting each of at least twooptical images of said alignment patterns, one of said optical imagescorresponding to said first wafer alignment pattern and said first maskalignment pattern, the other optical image corresponding to said secondwafer alignment pattern and said second mask alignment pattern, saidoptical images being converted into video signals by image pickupdevices incorporated in said units; means for causing said wafer-feedingtable to conduct step and repeat movements in accordance with a pitch ofchips contained in said wafer; and displacement detecting means fordetecting the relative displacement in the directions of X and Y axesbetween said wafer and said mask on the basis of said video signalsdelivered from said image pickup devices, said relative displacementbeing detected for at least three alignment patterns by operating saidmeans for causing said wafer-feeding table to conduct step and repeatmovements, said mask-feeding table and said wafer-feeding table beingrelatively moved on the basis of said relative displacement detected bysaid displacement detecting means to align said wafer and said mask witheach other, wherein, during the course of said adjusting, equal amountsof said projected light will be reflected off the first and second stepedges of each of said wafer alignment patterns due to the positionalrelationship of the longitudinal center axes of the wafer alignmentpatterns to the optical axis of the projection lens.
 7. A mask alignmentapparatus of projection type according to claim 6, wherein each of saidunits for scanning and detecting said optical images includes a scanningplate which is provided with a slit, is supported by a flat spring, andconducts the reciprocating motion, and said image pickup device fordetecting the optical image passing through said slit.